Impregnation of leather with polymer dispersion by application of pressure



United States Patent 3,245 832 IMPREGNATION 0F LEATHER WITH PULYMER DISPERSION BY APPLICATION OF PRESURE Roger H. Doggett, Natick, and Paul A. Plasse, Lexington, Mass, assignors, by mesne assignments, to Armour and Company, Chicago, IlL, a corporation of Delaware No Drawing. Filed Nov. 23, 1962, Ser. No. 239,763 2 Claims. (Cl. 117-142) This invention relates to leather impregnation, and more particularly to the treatment of sheepskin leather and the like which are unusual in that they have weak internal fiber strength tending to cause double hiding.

Sheepskin leather has been recognized as being considerably weaker in its internal fiber strength than many other like leathers. The grain and flesh sides of sheepskin leather are held together by fibers which are few in number and relatively weak and which provide a very porous internal structure. Because of such weak internal fiber formation, the use of sheepskin leather has been limited. If it were possible to bond effectively the flesh and grain sides of the sheepskin leather so as to provide uniform tensile and tear strength throughout the leather, the product could be employed more widely and effectively in such products as shoes, wearing apparel, and other commercial applications. Such bonding would eliminate the double hiding and consequent blistering that occurs when the grain and flesh sides pull apart under tensile stress and would widely increase the range of use of the material.

A primary object, therefore, of the present invention is to provide a method of processing sheepskin leather to render it uniform in cross-section with respect to tensile and tear strength and to eliminate double hiding. A further object is to provide a method for impregnating sheepskin leather or the like with elastomeric or resinous materials which effectively bond the grain and flesh sides terior of the sheepskin leather body can be filled and bonded to give it relatively uniform cross-sectional strength, while at the same time maintaining the leather in a flexible and pliable condition, this operation being carried on by working a very fine dispersion of a polymer into the leather or by soaking a monomeric material into the leather and then polymerizing the monomer in situ. For example, a polymer in the form of very small particles may be worked into the interior of the sheepskin leather by a pumping action while the leather is immersed in a liquid containing the polymer. By alternately pressing and releasing from pressure the leather piece, it is found that the polymer is drawn into the interior and is precipitated therein to strengthen the internal structure of the sheepskin leather. Alternatively, the monomer, such as a low-molecular-weight liquid, may be caused to impregnate the leather, as by soaking, and the monomer may then be polymerized so that the resulting polymer similarly strengthens the internal structure of the sheepskin leather.

In one embodiment of our invention, we provide a very fine dispersion of a resin having elastomeric properties, employing a liquid carrier, such as, for example, an aqueous bath. The dispersion preferably has stability toward acids, and particularly stable with respect to the acidity of the skins being treated. Small particle size is important in order to secure penetration of the leather, and we prefer a micro-particle size. The dispersion also should have stability with respect to aging during storage or shipment.

We prefer to have the dispersion in concentrated form, the concentration being adjusted so that it is almost to the point of precipitation when it comes in contact with the leather. The sheepskin leather is immersed Within the dispersion, and while immersed is subjected to multiple pressing and releasing so as to give a pumping action. When the leather is pressed, the liquids therein are expressed, and then after release of the pressure, liquids are again absorbed into the interior of the leather, thus drawing the polymer repeatedly into the leather interior. Such repeated pressing and releasing can be produced in a variety of ways. One simple method is to pass the leather between nip rollers which are immersed within the liquid so as to repeat the impregnation cycle for building up the resin deposit Within the matrix of the leather. Not only does the multiple pressing opera tion tend to draw the resin material into the leather, but also this action destroys the stability of the dispersion and causes the polymer to be precipitated within the interstices of the leather. In other words, by having the concentration of the dispersion near the point of precipitation, the nip rollers or other pressure means not only pump resin material into the interior of the leather, but also by destroying the stability of the dispersion precipitates the polymer where it is most needed.

Any suitable resin having elastomeric properties, as described above, may be employed. Such resins include polyacrylate latex, latex of vinyl acetate, styrene-butadiene small particle latex, acrylic latex, and a wide variety of polymeric resin materials known as acrylates, polyvinyl and vinylidene polymers, nitriled rubbers, etc. The solids content of the dispersion may vary widely depending upon the character of the leather being impregnated and the character of the impregnant. We prefer to employ a dispersion having a solids content of about 15- 25%; a 20% solids content has been found to be particularly satisfactory. By repeating the impregnation cycle, there is a marked increase in the pick-up of the resin in the interior of the leather until the interior is fairly well filled, and thereafter there is less pick-up, and, in fact, there is a point where further treatment begins to injure the leather from the standpoint of tongue tear test. Using nip rollers, it is found that about eight passes of the material between nip rollers well immersed within the dispersion bring about optimum filling or treatment.

By way of example, using polyacrylate latex (Hycar 2600X30), which has an average particle size of 1,800 angstroms, about 20% of the resin was deposited Within the sheepskin after 8 passes between nip rollers and the tensile strength increased to 28%. After 16 passes, there was an increase of tensile strength to 38%.

In order to replenish any oils that are lost during processing and to increase the tear resistance of the leather, we prefer to include an oil in the polymer bath. By incorporating in the impregnating bath an oil, such, for example, as emulsified sulfonated cod oil, we find that the stiffness of the material is minimized and that there is a marked increase in tea-r resistance. Further, the oil does not detract from the ability of the resin to overcome double hiding. Any suitable oil which is capable of replenishing lost oils may be employed.

In another embodiment of the invention, a monomeric material is soaked into the leather and then polymerized in situ. The monomer, such as a low-molecular-weight liquid, is caused to impregnate the leather, and after impregnation is polymerized so that the resulting polymer strengthens the internal structure of the leather.

If the temperature of reaction of the monomer may be too high and might damage the leather, a pro-polymer maybe formed, and while still a liquid introduced into the leather by soaking. For example, ethyl acrylate may be bulk polymerized to a controlled molecular weight, and the polymerization then stopped with a volatile inhibitor. The low-viscosity polymer, which is still a liquid, may be introduced into the leather by soaking and the leather wiped clean on the outer surfaces. The saturated leather may then be immersed in a warm water bath to remove the inhibitor and complete the polymerization of the impregnated material. The warm Water bath serves to exclude air from the sample, which would inhibit the polymerization, and to act as an ideal heat-exchange medium to promote the polymerization. The water also keeps'the leather in a soft, pliable state While the monomer is curing.

Specific examples illustrative of the invention may be set out as follows:

EXAMPLE I One-half of a sheepskin hide was tanned by the usual procedure, and then 7 test and 7 control strips were cut from the tanned hide. In order to provide the greatest degree of control, alternate strips were used as the test pieces, and the others were used as controls.

The control strips were observed for visual signs of double hiding. Then they were subjected to a tensile strength test and, following this, were again observed for visual signs of double hiding.

The test strips were treated by 8 passes through nip rolls immersed in 'a bath containing a solids dispersion of Hycar 2'600X-30 (polyacrylate latex). After such treatment, the strips were observed for visual signs of double hiding. They were then subjected to a tensile strength test and, following this, were again examined for signs of double hiding.

The following Table I indicates the value of treatment with Hycar 2600X- in overcoming double hiding:

One-half of a sheepskin was tanned by the usual procedure, and then 7 test strips and 7 control strips were cut from the hide, as in Example I.

The control strips were tested for tensile sterngth, and the averaged results are set out in Table II below.

The test strips were treated by passing them 8 times through nip rolls immersed in a bath containing a 20% 4 dispersion of Hycar 2600X-3(). The treated strips were tested for tensile strength, and the averaged results are set out in Table II below:

Table II Tensile Strength, lbs/sq. in. Average 01 7 samples Percent Increase in Tensile Strength No. samples that double ing Tensile Test Untreared Leather Leather treated withI-Iycar EXAMPLE III Another approach to obtaining a resin deposit is by the use of monomeric material which in its liquid form can be absorbed into the leather and followed by polymerization to form a solid polymer. Ethyl acrylate, which is readily reacted to form a soft pliable polymer, is suitable but the temperature of 90 C. required for polymerization damages the leather. To avoid subjecting the leather to this temperature, a pre-polyme'r is prepared by polymerizing a monomer to a controlled molecular weight and then stopping the polymerization with a volatile inhibitor. The low viscosity polymer, which is still a liquid, is then introduced into the leather by soaking, after which the leather is wiped clean on the outer surfaces. The optimum soaking time of the leather in the pre-polymer is about 30 minutes and is determined by the degree of densifying of the leather that results. The saturated leather is then immersed in a warm water bath to remove the inhibitor and complete the polymerization of the impregnating material. The warm water bath serves to exclude air from the sample which would inhibit the polymerization of the ethyl acrylate and also acts as an ideal heat-exchange medium to promote the polymerization. The water also keeps the leather in a soft, pliable state while the monomer is curing. The leather samples are then air-dried. The prepolymer of ethyl acrylate is prepared as follows: 7

The inhibitor is first removed from the monomer by either washing with a sodium-hydroxide sodium-chloride solution or by distillation of the monomer. The uninhibited monomer is then placed in a reaction flask and heated at a controlled rate. The reaction is carried out to a point at which the rate of temperature change of the reaction is quite marked due to the exothermic action. At this critical point of the reaction, the heat is turned off and the volatile inhibitor, such as cyclohexylamine, is introduced to terminate the polymerization.

The reaction product is a mixture of low-molecular- Weight ethyl acrylate polymers. If the reaction is stopped at the proper point to obtain a material for impregnation purposes, the polymer material will be a liquid and have a viscosity of approximately cps. at 25 C.

While in the foregoing specification we have set forth certain steps and ingredients in considerable detail for the purpose of illustrating the invention, it will be understood that such details may be varied widely by those skilled in the art without departing from the spirit of our invention.

of pressure.

2. The process of claim 1 in which oil is added to said liquid body to make up for oil lost by said leather in said pumping operation.

References Cited by the Examiner UNITED STATES PATENTS Baekeland 117-148 Harrison 1l8427 X Crowley 118-427 X Jennings 1177 X Kirby et a1. Cheronis 1l7--55 9/1956 Lawler 117-142 X 7/1957 Brown 117l42 X 7/1958 Riedel ll7163 X 8/1962 Haines et al. 117-119.6 4/1963 Hippen 25236O X FOREIGN PATENTS 7/ 1959 Australia. 12/ 1952 France.

OTHER REFERENCES Oehler et 211.: Treatment of Leather With Synthetic Resins, National Bureau of Standards, Research Paper RP 1951, vol. 42, January 1949, pp. 63 to 73.

Cheronis et 'al.: Deposition of Hydrophobic Polymers Within Leather, Leather Chemists Association, May 1949, pp. 282-87, 293-97, 307.

Flint, C. F.: The Chemistry and Technology of Rubber 0 Latex, p. 621, D. Van Nostrand Co., New York, 1938.

WILLIAM D. MARTIN, Primary Examiner. 

1. IN A PROCESS FOR UNIFYING SHEEPSKIN LEATHER HAVING FLESH AND GRAIN SIDES SEPARATED WITH A POROUS INTERIOR, THE STEPS OF IMMERSING THE LEATHER WITHIN AN EQUEOUS LIQUID BODY CONTAINING AN ELASTOMERIC RESIN MATERIAL CONCENTRATED TO A POINT NEAR ITS PRECIPITATION POINT, AND PRECIPITATING SAID MATERIAL WITHIN THE LEATHER SOLELY BY REPEATEDLY SUBJECTING SAID LEATHER TO PRESSURE AND RELEASE OF PRESSURE. 